It really depends on the player, really. I've been known to handle as much as two main bases, even three for Melee 400/400.
EDIT: Photon Cannon crawl? More like, start the game, siege with at least four zealots and make your units and cannons right at where your enemy is at.
EDIT: Photon Cannon crawl? More like, start the game, siege with at least four zealots and make your units and cannons right at where your enemy is at.